1. Field of the Invention
This invention relates to the electronics art, and more particularly to spark gap devices for the protection of circuit components against the effects of spurious voltage surges caused by lightning strikes, appliances, and other sources.
2. Description of the Prior Art
Spark gaps have been in service for many years in various circuits for various purposes. They are used principally to limit voltage excursions by shunting large voltage surges over a safe discharge pathway that bypasses other circuit components. Such voltage excursions can be caused by lightning strikes, appliances and other sources such as electrostatic discharge. Examples of electronic devices requiring voltage protection include cable television splitters, computers, cable modems, stereo and television equipment, to name but a few. All of these devices contain one or more printed circuit boards (PCBs) made from fiberglass or other suitable substrates with laminated copper traces formed thereon to electrically interconnect the mounted circuit components.
Unlike other surge protection devices such as metal oxide varistors, gas discharge tubes, thyristors and the like, spark gaps have the benefit of low cost and ease of manufacture. They can be formed on a PCB by positioning a pair of copper traces at a predetermined distance from each other to establish a controlled point of discharge for unwanted transient potentials that exceed a threshold breakover voltage. Examples of conventional PCB spark gaps are shown in U.S. Pat. No. 5,933,307 of West, U.S. Pat. No. 5,969,924 of Noble, and U.S. Pat. No. 4,586,105 of Lippmann et al. The '307 patent of West discloses a PCB spark gap comprising a pair of spaced copper traces separated by a slot formed in the PCB. The '924 patent of Noble discloses a PCB spark gap comprising a pair of serpentine circuit traces formed in a closely spaced nested arrangement. The '105 patent of Lippman et al. discloses a PCB spark gap comprising a pair of circuit traces formed with opposing pointed tips that are closely spaced from each other.
A disadvantage of PCB spark gaps is that the PCB surface and copper trace material degrades with each discharge due to vaporization of the copper material. Over time, the spacing of the spark gap increases due to the repeated degradation of the copper material. This changes the operating point of the spark gap by increasing the breakover voltage required to produce an arc across the gap. Insofar as the spark gap is built into the circuit board at the time of manufacture, it cannot be replaced or repaired, and the entire circuit board must ultimately be replaced if surge protection is to be maintained.
Various prior art solutions have been proposed to overcome this problem. In the '307 patent of West, it is stated that this problem can be addressed by increasing the circuit trace foil thickness, or by increasing the length of the slot (while retaining the same width), or by providing multiple spark gaps spaced serially from each other such that energy is dissipated in stages. Because the latter solutions are said to monopolize valuable PCB surface area, West proposes an alternative solution of forming part of a spark gap from surface mount components and solder fillets used to mount the components. Another solution to the problem of PCB spark gap degradation is found in British Patent No. GB2053579 of Griffiths et al. This patent discloses multi-fingered spark gaps in which the fingers are inclined to increase their length, thus enabling each finger to survive a longer period of burning.
It is to further improvement in the design of PCB spark gaps that the present invention is directed.